The present invention relates to a system for measuring the velocity of a car moving along a railway track by determining the location of the car along the railway track at two different points of time.
In the operation of today's modern classification yards, the speed of the cuts (i.e., a single car or group of cars travelling through the classification yard) directed to each of the particular storage tracks is controlled so that the cut when arriving at the storage track is travelling at a predetermined speed. That speed, herein called the entering velocity, is dependent upon, inter alia, the distance that the cut must travel in order to couple with the previous cut that entered the particular storage track. If a particular storage track has very few cars on it (presuming that all of the cars on the track are coupled together), it is necessary for the next cut entering that particular storage track to travel a relatively great distance and hence the cut when entering that storage track should be travelling at a greater entering velocity than would be desirable if the track was filled with more cars. Conversely, if the track is almost full, then the entering speed of the cut should be less so as to minimize the impact force against the last car already in the particular storage track. As is well known, the speed of the cuts can be controlled by the operation of the retarders in the classification yard.
The exit speed that the cut should have when it leaves the retarder, therefore, is dependent upon the distance from the retarder to the point of coupling in the storage track to which the cut is assigned. For proper coupling to occur, the car must be moving with sufficient momentum both to reach the last car of the previous cut and to close the coupling mechanism. In order to have such momentum the cut should generally be travelling at about 3 miles per hour upon impact. While this is the ideal coupling velocity, unfortunately several factors cause variations in the speed of the cuts. Such variations in the speed can lead either to damage to the cars of their lading if the cuts are travelling too fast or to failure of coupling if the cuts are travelling too slow.
The variations in speed can result from a plurality of different factors, in addition to the distance the cut must travel, such as the condition of the tracks or the forces of a strong wind. With respect to the condition of the tracks, this becomes especially significant where the classification yard is built upon a swamp, since the moisture and soft soil will affect the contour of the track. Attempts are often made to compensate for such variations by manually determining the deviation between the actual speed of a cut travelling along a particular storage track and the ideal speed for such cut. Upon determining the deviation, the information for controlling the retarder can be modified so as to compensate for the speed variations.
In order to determine the distance that a cut has to travel within a particular storage track before coupling, a measurement is made of the distance between the initial point of the storage track and the last car within that track. This measurement is often referred to as the DTC, i.e. the distance to coupling. In previously known systems, this measurement has been made through the use of either a radar system or through an impedance measuring system.
Two radar systems which are utilized for this purpose are shown by U.S. Pat. No. 3,377,587 to Nakahara et al. and U.S. Pat. No. 3,463,919 to DaRold et al. In the first of these two patents, the distance between two cars travelling along the same track is measured by the transmission of a radar signal along a specially built microwave waveguide positioned between the tracks. The signal is transmitted by one car towards the other car. The signal is then reflected by the second car back towards the generating point. The time between generation and receipt back of the radar signal provides an indication of the distance between the two moving vehicles. In the latter of the two patents, the same principle is utilized, but the radar signal is transmitted above ground without the use of any special conduits.
Radar system relying upon microwave waveguides are both extremely expensive and relatively complex both in setting up and in operation. In order to properly operate the radar system where the microwaves are transmitted along a waveguide, it is mandatory that the waveguides be properly aligned. While the alignment process has been relatively well developed, it is still a complex procedure, especially when the waveguides must be laid over great distances, as would be the case in a railway classification yard. Furthermore, the waveguides must remain in alignment. In all probability, the vibration caused by the rolling cuts along the track would tend to knock the waveguides out of alignment thereby rendering the system useless.
When a radar system is utilized where the microwaves are transmitted above the tracks, other problems occur. Such microwaves are generally transmitted through the use of a dish transmitter. Since the collimation of the beam is related to the size of the dish, in order to have relatively good resolution, the diameter of the dish must be fairly large. Of course, the larger the dish the larger the expense and the more impractical the system becomes. Furthermore, it is possible for the radar beams to bounce off other cars at adjacent locations if such cars are closer to the signal generating point. When this occurs, the system provides a measurement based on improper information.
Examples of the second type of system, an impedance measuring system, are shown in U.S. Pat. No. 3,342,989 to Dwyer et al., U.S. Pat. No. 3,619,604 to Auer et al. and U.S. Pat. No. 3,781,543 to Staples et al. In the system disclosed by each of these patents, a measurement is made of the attenuation in a signal which is transmitted along one rail of a particular track and reflected back along the other rail of that track. As shown by the patent to Auer et al., the signal is transferred from the first rail to the second rail by a shunt formed by a set of wheels and axle of a car located on the track. The attenuation of the signal is dependent upon the length of the rail along which the signal has travelled since the length of the rail varies the impedance of the current loop. Thus the longer the rail the greater the attenuation. The attenuation measurement, therefore, provides an indication of the distance that the signal has travelled which is indicative of the location of the car shunting the signal between the first and second rails of the particular track. The systems disclosed in the patents to Dwyer et al. and Staples et al. also rely upon impedance measurements. In the systems disclosed by both of these latter patents, multiplexers are provided so that the measurement system can be utilized for determining the location of the last car in each of a plurality of tracks.
In the impedance type measuring system such as shown in the above-noted patents, a current loop is formed between the measuring system, the railway track under consideration and the last car within the track. Since the resistance in the loop will depend upon the length of the track, the voltage varies with he length of the track. Prior to measuring the attenuation, however, it is necessary for all transients in the applied signal to die away so that a constant signal is transmitted along the tracks. Since the impedance measuring system requires a constant signal, there can be a delay of several seconds between the initial application of the signal and the time when a measurement can be made. Due to this delay, the amount of information, i.e., the number of tracks which can be tested within a set time, is significantly limited. Furthermore, such attenuation systems generally give a resolution only accurate to four car lengths. Since the retarder exit speeds of the cuts are dependent upon the measurement made by the system, it is obviously desirable to have the best resolution possible and often the poor resolution provided by an impedance measuring system can lead to either hard couplings and possible damage to the cars or stalling of the cars during the operation of the system.
More important than the problems exhibited by the radar and impedance measuring systems in rendering static measurements of the distance to coupling, it is extremely difficult with such systems to make any dynamic measurements relating to the movement of the car along the storage track. Due to the relatively poor resolution of such systems, if successive measurements are made at short time intervals, a forward moving car could appear to be either backing up or standing still. Consequently, in order to measure the speed of the cut moving along the storage track and hence coupling speed, there must be a significant time interval between each measurement of the location of the cut. The length of the time interval limits the number of speed measurements that can be made, which then means that it must be assumed that the deceleration of the cut as it travels along the storage track is constant. The coupling velocitys and decelerations of the cuts are used to control the retarder so as to minimize the deviations between the actual and ideal velocities.
The deceleration of the cut as it travels over the storage track, however, can vary due to several different reasons, such as tight gauge of the track, soft spots in the track bed and variations in the contour of the track. Such variations affect the dynamic characteristics of the cut as it travels along the storage track. Without being able to make a large number of speed measurements, it is impossible to accurately and automatically calibrate the system so as to minimize the speed deviations.